The present invention relates to fluid displacement apparatus and more particularly to a scroll-type machine especially adapted for compressing gaseous fluids and having intermediate relief valve means providing automatic variable pressure ratio characteristics to thereby improve efficiency, as well as protection against overcompression.
A class of machines exists in the art generally known as "scroll" apparatus for the displacement of various types of fluids. Such apparatus may be configured as an expander, a displacement engine, a pump, a compressor, etc. The present invention, however, is particularly applicable to compressors, and therefore for purposes of illustration is disclosed are in the form of a gaseous fluid compressor.
Generally speaking, a scroll apparatus comprises two spiral scroll wraps of similar configuration each mounted on a separate end plate to define a scroll member. The two scroll members are interfitted together with one of the scroll wraps being rotationally displaced 180 degrees from the other. The apparatus operates by orbiting one scroll member (the "orbiting" scroll member) with respect to the other scroll member (the "fixed" scroll member) to make moving line contacts between the flanks of the respective wraps defining moving isolated crescent-shaped pockets or chambers of fluid. The spirals are commonly formed as involutes of a circle, and ideally there is no relative rotation between the scroll members during operation, i.e., the motion is purely curvilinear translation (i.e. no rotation of any line in the body). The fluid pockets carry the fluid to be handled from a first zone in the scroll apparatus where a fluid inlet is provided, to a second zone in the apparatus where a fluid outlet is provided. The volume of a sealed pocket progressively changes as it moves from the first zone to the second zone. At any one instant in time there will be at least one pair of sealed pockets, and when there are several pairs of sealed pockets at one time, each pair will have different volumes. In a compressor the second zone is at a higher pressure than the first zone and is physically located centrally in the apparatus, the first zone being located at the outer periphery of the apparatus.
Generally, the greater the arcuate length of the scroll wrap the greater the possible total reduction in the volume of a pocket as it moves to the second zone (i.e. the greater the possible volume ratio); and the greater the volume ratio the greater the pressure ratio of the machine.
Two types of contacts define the fluid pockets formed between the scroll members: axially extending tangential line contacts between the spiral faces of the wraps caused by radial forces ("flank sealing"), and area contacts caused by axial forces between the plane edge surfaces (the "tips") of each wrap and the opposite end plate ("tip sealing"). For high efficiency, good sealing must be achieved for both types of contacts. In a conventional scroll compressor (i.e. one in which the wraps are involutes of a circle) good flank sealing requires that there be no relative rotation between the scrolls.
The concept of a scroll-type apparatus has been known for some time and has been recognized as having distinct advantages. For example, scroll machines have high isentropic and volumetric efficiency, and hence are relatively small and lightweight for a given capacity. They are quieter and more vibration free than many compressors because they do not use large reciprocating parts (e.g. pistons, connecting rods, etc.), and because all fluid flow is in one direction with simultaneous compression in plural opposed pockets there are less pressure-created vibrations. Such machines also tend to have high reliability and durability because of the relative few moving parts utilized, the relative low velocity of movement between the scrolls, and an inherent forgiveness to fluid contamination.
A scroll compressor is a positive displacement fixed volume-ratio machine; at the suction inlet a given volume of a gaseous fluid is sealed off and compressed to a final volume at which it is discharged. Because it has a fixed volume ratio, it also has a fixed pressure ratio. The pressure of the final compressed volume, and for that matter all intermediate volumes between initial seal-off and the final compressed volume, is determined substantially by two factors: (1) the pressure of the initial suction volume at seal-off, and (2) the volume reduction during compression. Thus, the pressure of the initial charge will rise to whatever pressure is dictated by the volume reduction during compression. This type of compression process can place severe limitations on the efficiencies, operating life, and actual pressure ratios attainable with the apparatus.
After a scroll compressor in a refrigerating system has been operating and then shut down, the pressure differential between the sealed-off pockets or chambers dissipates via leakage, heat transfer, etc. until the entire system becomes pressure equalized at a pressure somewhere between the suction and discharge pressure. Upon restartup, the equalization pressure, which is substantially higher than the suction pressure at which the compressor was designed to normally operate, becomes the initial suction pressure. During initial restartup, therefore, because the compressor has a fixed design pressure ratio, abnormally high chamber pressures can be developed. This condition can result in over-loaded bearings, over-stressed components, very high starting torque and even stalling of the machine. The high pressure condition is most severe at the beginning of restartup and diminishes as the suction pressure is "pulled down" to the normal operating suction pressure. For example, if a compressor has a 10:1 pressure ratio under design conditions, suction gas at 30 psi will be compressed to 300 psi. On the other hand, if the equalization pressure of the same system is 135 psi, then on start-up pressures in the magnitude of 1350 psi will be developed. In high pressure-ratio scroll compressors the problem is aggravated. Thus, the present invention is especially suited for use with high pressure-ratio machines such as that disclosed in my copending application entitled Scroll-Type Machine filed of even date.
Scroll compressors are designed to operate in a system having a design operating pressure or pressure range. The purpose of the compressor is to compress a gaseous fluid at suction pressure to the system operating pressure. Since the basic scroll compression process is a fixed pressure-ratio process, operation away from the design pressure ratio of the scroll results in compression inefficiencies. For example, if the scroll compressor is coupled with a system which is functioning at an operating pressure which is lower than the scroll compressor design pressure-ratio, the pressure in the final compressed volume just prior to discharge will be higher than the actual pressure in the system. Thus, on discharge the over-compressed gas will expand until the actual system pressure is reached, thereby causing inefficiency due to the work lost in over-compression.
It is therefore desirable to provide a means for reducing or eliminating over-compression when the system operating pressure is lower than the scroll compressor design pressure-ratio, thereby increasing compression efficiency. It is also desirable to provide a means to reduce or eliminate destructively high pressures which can occur upon restart-up.
Another advantage of the present invention is that it permits the design of a compressor having a greater than normal pressure ratio, for the purpose of avoiding those periods when system pressure exceeds the normal design pressure of the compressor, which would result in inefficient undercompression and reexpansion of the compressed gas.
The scroll-type apparatus of the present invention incorporates a pair of intermediate dump or pressure relief valves in either the fixed or orbiting scroll member which will automatically open the corresponding compression chamber to discharge plenum when the pressure in the chamber becomes greater than the system pressure seen by the discharge plenum, thereby stopping compression and dumping the gas to discharge. The valves are constructed of a lightweight material for quick action and good sealing, and are configured so that no reexpansion space is created in the fluid chambers, thereby maintaining efficiency. In this manner inefficient over-compression and excessively high pressures, along with the attendant bearing loads, starting torques, stresses, etc., are virtually eliminated, in simple and highly efficient manner.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.